This invention relates to the field of communications, and in particular to the processing of multiple asynchronous spread-spectrum communications.
Spread-Spectrum techniques are used to modulate an information signal such that the modulated signal appears as noise. The information is modulated by a pseudo-random sequence of bits, and can be demodulated, or despread, using the same pseudo-random sequence. This modulation is commonly referred to as Direct-Sequence Spread Spectrum (DSSS). The modulated signal is spread across a bandwidth that is substantially larger than the bandwidth of the information signal, and has the apparent effect of increasing the noise-floor of receivers that receive this signal. Applying the same pseudo-random sequence to the modulated signal allows the information signal to be detected within this apparent noise.
Code Division Multiple Access (CDMA) is a commonly used spread-spectrum communications technique wherein the information signal is encoded by one of many code sequences before it is transmitted. The received signal is decoded by the same code sequence to reproduce the original information signal. Transmissions from multiple transmitters can be simultaneously communicated via a common frequency channel by employing different code sequences for each transmitter, provided that the code sequences have particular uniqueness characteristics. The uniqueness characteristics of acceptable codes substantially guarantee that a coherent output will only be produced when the received signal corresponds to a signal that is encoded using the same code sequence. Signals that are not encoded using the same code sequence as the decoding code sequence exhibit minimal coherence and are filtered out as noise signals. In a conventional CDMA system, such as a cellular telephone network, the network controller allocates and deallocates code sequences on demand, so that each of the transmitters can transmit over the same network without interference from other transmitters.
A significant characteristic of a pseudo-random spread spectrum code is that a coherent output is only produced when the decoding code sequence is applied substantially in phase with the encoding code sequence. If the received signal is decoded with a code-phase that is out of phase with the transmitter, and the code is a proper pseudo-noise code, having the above defined uniqueness characteristics, then the decoding of this out-of-phase signal produces a noise output. U.S. Pat. No. 5,537,397, “SPREAD ALOHA CDMA DATA COMMUNICATIONS”, issued Jul. 16, 1996, to Norman Abramson, and incorporated by reference herein, discloses a technique that uses this phase-dependency characteristic to allow multiple transmitters to use the same code concurrently. As in the conventional CDMA system, the network controller provides an allocation to each transmitter, but in the referenced patent, each transmitter is allocated a different time-slot, or code-phase, rather than a different code. The controller instructs each transmitter to advance or delay its transmission, so that its signal is received at the receiver with a code-phase that is sufficiently different from the code-phase of other transmitters. In this manner, although each of the transmitters and the receiver use the same code, each transmitter provides a “different” (phase-shifted) code to the receiver, relative to the particular code-phase of the decoder at the time of decoding.
The aforementioned prior art technique requires a unique identification of each mobile transmitter, because the communication of each allocated code or code-phase must be directed to the appropriate transmitter. Each mobile transmitter must also be equipped with a receiver, to receive and process the communicated phase allocation. In conventional cell-phone systems, each base station transmits a pilot signal that the mobile systems use to synchronize their code-phase to the base station's phase. Due to propagation delays and other factors, this synchronization is a ‘coarse’ synchronization that allows the base station to locate the transmissions within a relatively narrow timespan of when the “in-phase” transmissions should be received at the base-station. Once the in-phase signal is received at the base station, a phase-locked loop is used to assure that the base station receiver remains in sync with the mobile transmitter, to compensate for any differences between the transmitter's frequency and the receiver's frequency. That is, a separate phase locked loop is required for each currently active transmitter.
The requirement to dedicate a receiver to each active transmitter requires the use of multiple receivers operating in parallel, which can become costly, particularly if the receiving system needs to be designed for a ‘peak’ number of simultaneously transmitting transmitters.
U.S. Pat. No. 6,128,469, “SATELLITE COMMUNICATION SYSTEM WITH A SWEEPING HIGH-GAIN ANTENNA”, issued 3 Oct. 2000 to Ray Zenick, John Hanson, Scott McDermott, and Richard Fleeter, and U.S. Pat. No. 6,396,819, “LOW-COST SATELLITE COMMUNICATION SYSTEM”, issued 28 May 2002 to Richard Fleeter, John Hanson, Scott McDermott, and Ray Zenick, and U.S. Pat. No. 6,317,029, “IN-SITU REMOTE SENSING” issued 13 Nov. 2001 to Richard Fleeter, disclose systems and methods that facilitate the reception and processing of messages from a large number of preferably low-cost transmitters, and are each incorporated by reference herein. For example, a large number of IC chip-size transmitters may be released from an aircraft that overflies a hurricane or forest fire. These transmitters may be configured to periodically or randomly transmit their location and the atmospheric conditions at their location, such as temperature, pressure, moisture content, and so on. A receiving system receives and processes the transmissions from these many devices and provides temperature and pressure profiles, changes and trends, predictions, and the like. Such systems require simple, low-cost, and efficient transmitters.
Conventionally, communication messages (whether spread-spectrum or not) include a ‘preamble’ (also known as a ‘front porch’) that is prepended to the information content of the message to facilitate all necessary synchronization of the receiver to the transmitted signal and other required functions, such as identifying the beginning of each message, identifying bit polarity, aligning data packets, and so on. However, in the above referenced applications, the transmitters are often required to be portable and battery-operated. In applications where the information content is small, such as a report of the security of a lock on a transport container, the transmission of the preamble with each short message often consumes more energy than the transmission of the information. It also occupies a disproportionate amount of total system bandwidth, reducing the maximum number of possible users in the system: time spent transmitting non-information-containing preamble bits is time another device cannot spend transmitting useful data. In an information theory sense, the most useful communication is a highly stochastic bit sequence, yet the purpose of a preamble is to be a completely nonstochastic (predictable) bit sequence.
It would be advantageous to provide a receiving system that is configured to distinguish among transmissions from a plurality of transmitters that share a communication channel and use a common DSSS code sequence without requiring a preamble with each message. Correspondingly, it would be advantageous to provide a DSSS transmitter that does not transmit a preamble with each message. It would also be advantageous to allow a single receiving system to search for all present transmitters, to avoid having to provide multiple parallel-operating receivers.
These objects and others are achieved by providing a receiving system that allows for the detection of a transmission at any point in time during the transmission, thereby avoiding the need to identify the start of the transmission during the transmission-detection process. An input buffer captures the transmissions on a communication channel using a moving time-window. A detector processes a time-slice from the input buffer and identifies all of the simultaneously transmitting transmitters during that time-slice. As each transmitter is identified, the demodulator traces back-in-time to identify where the message can first be detected in the input buffer. The transmission includes suitable characteristics to facilitate detection and demodulation of the message content, but need not contain a preamble to allow the detection process. Provided that the time-window of the input buffer is sufficiently large to include the time required to find all of the transmissions in the time-slice and includes the time back to the previous time-slice, no information will be lost.
Throughout the drawings, the same reference numerals indicate similar or corresponding features or functions. The drawings are included for illustrative purposes and are not intended to limit the scope of the invention.